Update documentation for changing isa method to arrow function (#164)

Author: trivikr

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/StructureGenerator.java

@@ -77,9 +77,7 @@ public void run() {
      * }
      *
      * export namespace Person {
-     *   export function isa(o: any): o is Person {
-     *     return __isa(o, "Person");
-     *   }
+     *   export const isa = (o: any): o is Person => __isa(o, "Person");
      * }
      * }</pre>
      */
@@ -138,10 +136,8 @@ private void renderNonErrorStructure() {
      *   resourceType: string | undefined;
      * }
      *
-     * export namespace Person {
-     *   export function isa(o: any): o is NoSuchResource {
-     *     return __isa(o, "NoSuchResource");
-     *   }
+     * export namespace NoSuchResource {
+     *   export const isa = (o: any): o is NoSuchResource => __isa(o, "NoSuchResource");
      * }
      * }</pre>
      */